An external auditor performs an audit, in accordance with specific laws or rules, of the financial statements of a company, government entity, other legal entity, or organization, and is independent of the entity being audited. Users of these entities' financial information, such as investors, government agencies, and the general public, rely on the external auditor to present an unbiased and independent audit report.

The manner of appointment, the qualifications, and the format of reporting by an external auditor are defined by statute, which varies according to jurisdiction. External auditors must be members of one of the recognised professional accountancy bodies. External auditors normally address their reports to the shareholders of a corporation. In the United States, certified public accountants are the only authorized non-governmental external auditors who may perform audits and attestations on an entity's financial statements and provide reports on such audits for public review. In the UK, Canada and other Commonwealth nations Chartered Accountants and Certified General Accountants have served in that role.

For public companies listed on stock exchanges in the United States, the Sarbanes-Oxley Act (SOX) has imposed stringent requirements on external auditors in their evaluation of internal controls and financial reporting. In many countries external auditors of nationalized commercial entities are appointed by an independent government body such as the Comptroller and Auditor General. Securities and Exchange Commissions may also impose specific requirements and roles on external auditors, including strict rules to establish independence.